The invention relates to a doctor blade for wiping excess printing ink off the surface of a printing form.
A doctor blade of this type, which is used in the gravure printing process, for example, is disclosed by U.S. Pat. No. 4,184,429. The doctor blade comprises a rear doctor blade part, which is installed in a holder, and a front leaf, which rests on the surface of the printing form with a certain pressure. The leaf is significantly thinner than the rear part of the doctor blade (its thickness is approximately one half of the thickness of the rear part of the doctor blade). The advantage of this doctor blade is that when the front contact zone, which interacts with the surface of the printing form, wears, this contact zone remains as far as possible unchanged and unimpaired in terms of its size and geometry needed for the required print quality, until the entire width of the leaf has been used up or worn away. The constant geometric relationships in the area of contact, and hence also the constant doctor blade pressure, make manual readjustment of the contact pressure superfluous and permit constant print quality.
Even in the case of these doctor blades, however, there is the risk that the surface of the printing form, the uppermost chromium layer in the case of gravure printing forms, will be worn away or damaged by the doctor blade. In addition, the doctor blade is also subjected to wear. The tribology in relation to a steel doctor blade/chromium layer has always been a problem. In order to eliminate these disadvantages, DE-A-28 17 964 discloses a proposal to embed a lubricant in the doctor blade itself, this lubricant appearing in the contact zone and reducing the friction between the doctor blade and the printing form surface. This solution has the disadvantage that the properties of the doctor blade, for example its resilient behavior, are impaired by the embedded lubricant. In addition, this solution is difficult and expensive to produce.
In order to reduce the wear on the doctor blade, it is also known to coat the doctor blade with wear-resistant material in the region of the leaf. However, this solution increases the risk of wear or damage to the surface of the printing form.
The same problems also arise in the case of narrow doctor blades, which are formed from a thin metal plate which is firmly clamped between two supporting plates and has a constant thickness over the entire width of the doctor blade.
The present invention is based on the object of providing a doctor blade in which the wear on the surface of the printing form and the doctor blade wear are reduced considerably, without the properties of the doctor blade being impaired.